A display panel capable of detecting a position on the screen indicated by a pointer such as a user's finger or a pen (hereinafter sometimes referred to as “touch position”) is widely known. In order to detect the touch position, the display device is provided with a touch panel with a touch sensor. There is a projected capacitive touch sensor as a type of the touch sensor. In the projected capacitive method, even if the surface on the user side of the touch screen, in other words, the surface on the side where the display device is observed (hereinafter sometimes referred to as “observation surface” is covered with a protective plate such as a glass plate of about several mm in thickness, the touch position can be detected. Therefore, excellent robustness can be obtained. In addition, since there is no moving part, a long life can be obtained.
The projected capacitive touch sensor includes row-direction-detecting wiring lines for detecting a coordinate of the touch position in the row direction, and column-direction-detecting wiring lines for detecting a coordinate of the touch position in the column direction (see, for example, Patent Document 1). In the following description, the row-direction-detecting wiring lines and the column-direction-detecting wiring lines may be together referred to as “detecting wiring lines”. Patent Document 1 discloses a touch pad system corresponding to a touch panel. The touch pad system disclosed in Patent Document 1 includes a first series of conductor elements formed on a thin dielectric film, and a second series of conductor elements formed on the first series of conductor elements with an insulating film interposed therebetween as the detecting wiring lines for detecting the electrostatic capacitance (hereinafter sometimes simply referred to as “capacitance”). In a plan view, the first series of conductor elements and the second series of conductor elements intersect each other, but there is no electrical contact at these intersections. The touch position is specified by the detection of capacitance (hereinafter sometimes referred to as “touch capacitance”) formed between a pointer such as a finger and conductor elements being a touch sensor with a detection circuit. In addition, relative values of the detection capacitance of two or more conductor elements allow the touch position between the conductor elements to be interpolated.
In the following description, a member in which column-direction-detecting wiring lines and row-direction-detecting wiring lines are arranged on a transparent dielectric substrate is referred to as a “touch screen”, and a device in which a detection circuit is connected to the touch screen is referred to as a “touch panel”. In addition, in the touch screen, a region in which a touch position can be detected is referred to as “operation area” or “detectable area”.
In the operation area of the touch screen, it is necessary to densely arrange the detecting wiring lines in the operation area in order to detect all the touch positions of the pointer. Since the dense detecting wiring lines are easy for the user to visually recognize, the dense detecting wiring lines are not preferable from the viewpoint of the image quality of the display panel. If the detecting wiring lines are made of a transparent conductive film such as indium tin oxide (ITO), the possibility that the detecting wiring lines are visually recognized by the user is reduced. However, since the transparent conductive film has relatively high electrical resistance (hereinafter sometimes simply referred to as “resistance”), the transparent conductive film is disadvantageous for increasing the size of the touch screen. In addition, corrosion relatively easily occurs between the transparent conductive film and other metal wiring lines, and as a result, the wiring lines may be broken. Therefore, when a touch screen is attached to a liquid crystal display (LCD) to be used, there is a problem with the stability of the wiring line to humidity and water drops.
In order to avoid the above problem, it is conceivable to use a low resistance metal material such as silver or aluminum as the material of the detecting wiring line. Using a wiring line made of a metal material (hereinafter sometimes referred to as “metal wiring line”) as the detecting wiring line allows the resistance of the detecting wiring line to be lowered. Since the metal wiring line is opaque, the metal wiring line is easily visually recognized by the user as described above. As a method of reducing the case of visibility by the user, there is a method of thinning metal wiring lines (for example, mesh wiring). Patent Document 2 discloses a projected capacitive touch screen with metal line wiring lines.
Densely arranging the thinned metal mesh wiring lines on the operation area of the touch screen causes a problem that the parasitic capacitance between the column-direction-detecting wiring lines and the row-direction-detecting wiring lines (hereinafter sometimes referred to as “line capacitance”) greatly increases. As a result, for example, an adverse effect such as an increase in wiring delay or an increase in noise is caused.
The wiring delay can be alleviated to a certain extent by the resistance of the wiring lines being reduced. A technique for reducing the resistance of the wiring lines in order to alleviate the wiring delay is disclosed in, fir example, Patent Document 3. In the touch screen disclosed in Patent Document 3, each of the row-direction-detecting wiring lines and the column-direction-detecting wiring lines is set to form a zigzag pattern in which linear-shaped and thin line-shaped metal wiring lines are connected. Thus, both reduction in resistance and reduction in line capacitance are compatible with each other. In addition, in the touch screen disclosed in Patent Document 3, a plurality of row-direction-detecting wiring lines extending roughly in the row direction are electrically connected to form a row-direction bundled wiring line, and a plurality of column-direction-detecting wiring lines extending roughly in the column direction are electrically connected to form a column-direction bundled wiring line. This makes it possible to uniformly detect the touch capacitance including the capacitance between the pointer such as a finger and the row-direction-detecting wiring lines and the capacitance between the pointer and the column-direction-detecting wiring lines.
The touch panels disclosed in the above Patent Documents 1 to 3 can be manufactured separately from the display panel that performs the display function. In this case, since a transparent substrate other than the substrate for the display panel is required, the thickness and weight of the display device increase. This is unpreferable specifically in mobile devices and the like. For this reason, Patent Documents 4 and 5 disclose methods of integrating the touch screen function in the inside or on the surface of the display panel. The method of integrating a touch screen inside a display panel as in Patent Document 4 is referred to as an in-cell method, and the method of integrating a touch screen on the surface of a display panel as in Patent Document 5 is referred to as an on-cell method. In the in-cell method, the signal due to capacitive coupling between the finger and the detecting wiring lines is poorly generated, and the accuracy of the touch detection may be deteriorated. In the on-cell method, such accuracy deterioration can be suppressed.
According to Patent Document 6, metal fine wiring line is applied to the on-cell method. The mesh-shaped detecting wiring lines are formed on one side or both sides of the color filter substrate so as to coincide with the formation position of the light-shielding portion formed in a pattern shape. Thus, improvement in accuracy of touch detection and thinning and weight-reducing thereof are made compatible. When detecting wiring lines are provided only on a surface being one side of the color filter substrate, the surface on which the light-shielding portion is provided, the distance between the detecting wiring lines and the finger becomes larger, and the distance between the detecting wiring lines and the pixel electrodes becomes smaller. Therefore, the detecting wiring lines are susceptible to the influence of the electric field for driving the liquid crystal. Thus, the accuracy of the touch detection deteriorates similarly to the in-cell method. According to Patent Document 7, a first detection electrode is formed on a first surface of a color filter substrate, and a second detection electrode is formed on a second surface opposite to the first surface.